Infrared spectroscopy is a technique for analyzing the chemical makeup of a sample and is widely used in many applications, such as medical diagnostics, petroleum exploration, environmental health monitoring, and drug testing.
Identifying and quantifying the chemical makeup of a test sample spectroscopically is enabled by the fact that atoms are not held rigidly apart in a molecule. Instead, they can move, as if they are attached by a spring that can bend and stretch. If a bond between a pair of atoms is subjected to radiation of a particular energy, it can absorb the energy and the bond can move from a first vibrational state to the next higher vibrational state. The specific wavelengths at which such absorption takes place is determined by the shape of the molecular potential energy surfaces, the masses of the atoms and their coupling, which are specific to every molecule. As a result, the set of wavelengths at which radiation is absorbed by a material (its set of “absorption peaks”) is indicative of the chemical makeup of that material. As a result, these absorption peaks are often referred to as “finger-print absorption peaks.”
In transmission infrared spectroscopy, infrared radiation is directed through a test sample and detected after it has passed through the material. As the radiation passes through the sample, each chemical constituent of the material selectively absorbs radiation at its characteristic wavelengths, thereby imparting chemistry-dependent spectral information on the detected radiation. The positions, magnitudes, and inflections of the spectral peaks in the output signal provide a “spectral fingerprint” that is then used to estimate the chemical makeup of the sample.
The mid-infrared (MIR) spectral range (defined herein as the wavelengths within the range of approximately 2.5 microns to approximately 12.5 microns) represents a particularly information-rich spectral region because of the wealth of absorption peaks that exist within it for most chemicals. The MIR spectral range, therefore, is an attractive operating range for infrared spectroscopy. As a result, several MIR spectrometers have been disclosed in the prior art, such as those disclosed by Muneeb, et al., in “Demonstration of Silicon on insulator mid-infrared spectrometers operating at 3.8 microns,” Optics Express, pg. 11659 (2013) and those disclosed by Shankar, et al., in “Silicon photonic devices for mid-infrared applications,” Nanophotonics, Vol. 3, pg. 329 (2014). Generally, known MIR spectrometers are based on a wavelength dispersion element (e.g., a prism or diffraction grating) that spatially spreads the spectrum of interrogating light across a region of a test sample. The light passes through the test sample and is detected by an array of substantially identical detectors—either semiconductor detectors that measure a photoelectric effect, or bolometers that measure changes in temperature due to absorption of incident radiation. It should be noted that precise alignment between the source, test sample, and detector array is required to enable proper registration between the output signal of each detector and the wavelength of light believed to be incident upon it.
Unfortunately, prior-art MIR spectroscopy systems have many drawbacks. First, conventional MIR spectrometers suffer from narrow bandwidth. Second, their wavelength resolution is too coarse to effectively identify many chemicals—typically due to an insufficient number of detectors. Third, most prior-art MIR spectrometers require external sources and detectors, making them quite complex, difficult to align and keep aligned, and subject to failure due to environmental shock and vibration. Fourth, scatter of radiation within the test sample can lead to cross-talk between detector pixels, since the output signal of each bolometer is merely a function of whatever radiation is incident upon it.
Further, prior-art spectrometers based on semiconductor detectors, which measure photoconductivity or diode current changes, have additional drawbacks. In a semiconductor detector, incident photons excite valence electrons to the conduction band to give rise to a macroscopically detectable electric current. Commonly used mid-IR detectors include mercury cadmium telluride, gallium tin, indium tin, or germanium, each of which has a relatively small electrical bandgap. Because of these small bandgaps, these semiconductors typically have high leakage currents that compromise the signal-to-noise performance of the detectors. To mitigate leakage current, they are normally cooled to below ambient temperature during operation. The need to operate at low temperature, however, severely limits their portability, as well as their use for biological analysis.
While conventional bolometers represent an attractive alternative to semiconductor MIR detectors in many cases, they are not without their own disadvantages. A conventional bolometer detects incident radiation by absorbing the radiation and converting its energy into heat that manifests as a change in temperature. Bolometers typically require materials with large thermo-resistance coefficients, such as vanadium oxide, as well as long light-interaction lengths to enable sufficient absorption for detection. Mid-IR sensitive bolometers observe the conductivity change of thermo-resistive materials deposited onto thermally isolated membranes and can function at room temperature; however, because a bolometer inherently functions as a heat detector, bolometer-based prior-art systems are highly sensitive to changes in ambient temperature.
The need for a practical, high-sensitivity, robust MIR spectrometer remains unmet in the prior art.